1. Technical Field
The present invention relates to a multilayer band pass filter suitable for use in mobile communication instruments such as portable telephones, cordless telephones and the like, the multilayer band pass filter comprising a plurality of deposited dielectric layers made of ceramics or the like, a core conductor forming an inductor between each pair of adjacent dielectric layers or on each dielectric layer, and electrodes forming a capacitor.
2. Background Information
One conventional multilayer, band pass filter is described in Japanese Patent Laid-Open No. Hei 3-265205.
Such a deposited layer type band pass filter comprises a first ceramics layer, a core conductor, a second ceramics layer, a zero point forming capacitor electrode, a third ceramics layer, first electrodes, a fourth ceramics layer, second electrodes, a fifth ceramics layer and shield electrodes, each of which being deposited in the described order starting from the lowermost layer. The multilayer band pass filter also comprises a pair of input/output terminals formed at opposing sides therein. Each of the input/output terminals is connected to one of the two first electrodes. The first and second electrodes oppose each other having the forth ceramics layer positioned therebetween to form an input/output capacitor. The ends of the second electrodes are respectively connected to the open ends of the core conductors through other side electrodes. The second electrodes are positioned opposing the shield electrodes through the fifth ceramics layer to form resonance capacitors, each of which being connected in parallel to the corresponding core conductor.
The first electrodes both on the input and output sides are arranged opposite to the zero point forming capacitor electrode to form a zero point forming capacitor. The zero point forming capacitor provides a kind of trap circuit which has a given frequency to control a disturbing signal from any adjacent channel or unnecessary radiations at the zero point (fn.sub.1). In the prior art, the zero point forming capacitor is entirely enclosed by ceramics.
On manufacturing such a multilayer band pass filter, there are produced manufacturing variabilities in the area of the zero point forming capacitor electrode, the area of the first electrodes, the distance between the adjacent electrodes, the dielectric constant of the ceramics and other factors. The manufacturing variabilities result in a variability in the capacity of the zero point forming capacitor. This in turn results in variabilities in the characteristics of the multilayer band pass filter, such as deviation of zero point frequency, less reduction of zero point level and other factors.
In the prior art, it is practically impossible to regulate the capacity of the zero point forming capacitor after the multilayer band pass filter has been baked. More particularly, the main factors for changing the capacity of the zero point forming capacitor are the area of the opposed faces in the zero point forming capacitor electrode and the first electrodes, the distance between the adjacent electrodes and the dielectric constant in the ceramics. However, the zero point forming capacitor electrodes cannot be trimmed since it is enclosed by the ceramics layers. The necessary parts of the first electrodes cannot be trimmed either since the faces of the first electrodes opposite to the zero point forming capacitor electrode are similarly enclosed by the ceramics layers. Thus, the area of the opposed faces in the zero point forming capacitor electrode and the first electrodes cannot be changed. Further, the distance between the zero point forming capacitor electrode and the first electrodes, as well as the dielectric constant in the ceramics layers, cannot be changed after the multilayer band pass filter has been baked. In the prior art, therefore, the capacity of the zero point forming capacitor cannot be regulated after the multilayer band pass filter has been baked. This is true of the regulation of the other internal capacitors.
The problems mentioned above are similarly raised by multilayer band pass filters other than the multilayer band pass filter.
FIGS. 22A and 22B show one example of the multilayer band pass filters constructed in accordance with the prior art. FIG. 22A is a longitudinal cross-section of the multilayer band pass filter, while FIG. 22B is a right-hand side view of the multilayer band pass filter. Namely, FIG. 22A is a view as viewed along a line I--I from a direction or arrow in FIG. 22B. In FIG. 22B, the soldering parts will be omitted.
The multilayer band pass filter F103 of the prior art shown in FIGS. 22A-22B comprises a resonance circuit consisted of an inductor and a resonance capacitor. The inductor is formed by a core conductor 110 while the resonance capacitor is formed by a resonance capacitor electrode 120, a ground electrode 121 and ceramics 130. The open end 111 of the core conductor 110 is connected to the electrode end portion 120e of the resonance capacitor electrode 120 through a connecting surface electrode 140. The connecting surface electrode 140 is provided on the surface (side) 130 of the multilayer band pass filter F103. The short-circuit end 112 of the core conductor 110 and the ground electrode 121 are connected to another ground electrode 122 which is formed on the surface of the filter F103. A ground pattern 151 is formed on a mother board 150 and connected to the ground electrode 122 through a soldering part.
On production of the filter F103, some thin green sheet (ceramics before baked) are deposited to form a sub-assembly. An electrically conductive paste is printed to form the core conductor 110 over which a green sheet is deposited. Over this green sheet, an electrically conductive paste is printed to form the resonance capacitor electrode 120 over which a green sheet is deposited to form a final assembly. An electrically conductive paste (silver paste) forming the connecting surface electrode 140 and ground electrode 122 is transferred and printed on the final assembly after it has been baked.
Before the conductive paste for the connecting surface electrode 140 is to be transferred and printed onto the filter F103, a drum having a longitudinally extending groove formed therein is provided, and the conductive paste is filled in the groove. The filter F103 is carried to, and positioned at, a location where the face of the filter F103 in which the connecting surface electrode 140 is to be formed opposes the filled groove. Thereafter, the transferring and printing step will be carried out. The connecting surface electrode 140 is formed to extend on the face 131 of the filter F103 from the uppermost end thereof to the lower end 141.
FIG. 23 is a view showing an equivalent circuit in the aforementioned structure of the prior art.
A resonance circuit is defined by an inductor L101 and a resonance capacitor C101, and another resonance circuit is formed by an inductor L102 and a resonance capacitor C102. The inductor L101 is formed by the core conductor 110. The resonance capacitor C101 is formed by the resonance capacitor electrode 120, the ground electrode 121 and the ceramics 130. The inductor L102 and resonance capacitor C102 are formed in the similar manner. In FIGS. 22A-22B, the structure relating to input/output capacitors C103 and C104 will be omitted.
In the prior art, the band pass filter F103 is fixed to the mother board 150 through soldering, but the connecting surface electrode 140 is not connected to the pattern on the mother board 150. The lower end 141 of the connecting surface electrode 140 is separated from the pattern of the mother board 150. If a ground pattern 152 exists near the lower end 141 of the connecting surface electrode 140, capacitors C105 and C106, as shown in FIG. 23, will be formed between the lower end 141 of the connecting surface electrode 140 and the ground pattern 152. Capacitors C105 and C106 cause the resonance point (resonance frequency) in the resonance circuit to be changed. In addition, if a wiring pattern, through which signals pass, exists near the lower end 141 of the electrode, any unnecessary coupling will be created to disturb the operation of the entire system.
Therefore, the prior art requires that when the band pass filter F103 is to be mounted on the mother board 150, the wiring patterns such as ground pattern and other patterns are located remote from the lower end 141 of the connecting surface electrode 140. This makes the design of circuit cumbersome.
The problems mentioned above are also raised by filters other than the band pass filter.
The present invention has an object to provide a multilayer band pass filter comprising an extension electrode connected to an input/output terminal, a capacitor electrode arranged opposed to the extension electrode through ceramics to form an input/output capacitor, core conductors forming an inductor, the open end of which is connected to the capacitor electrode, a ground electrode positioned opposite to the capacitor electrode to from a resonance capacitor with the zero point forming capacitor, whereby the capacity of the zero point forming capacitor can be regulated after the multilayer band pass filter has been baked.
The present invention has another object to provide a multilayer band pass filter and filter circuit comprising first and second extension electrodes respectively connected to two input/output terminals, a capacitor electrode arranged opposed to the first and second extension electrodes through dielectric to form first and second input/output capacitors, core conductors forming an inductor, the open end of which is connected to the capacitor electrode, a ground electrode connected to the capacitor electrode to form a resonance capacitor therebetween, and a zero point forming capacitor, whereby zero points can be formed below and above a given range.
The present invention has still another object to provide a multilayer filter which can cause the resonance point in the resonance circuit to be influenced by the wiring patterns on the mother board, and which can prevent the operation of the entire system from be disturbed by any unnecessary coupling with the wiring pattern through which signals pass.
The present invention has a further object to provide a filter and a method of producing such a filter, where the plated film thickness of the filter is substantially uniform even when electrodes which are to be electrically plated are different in area from one another.
Preferably, the multilayer band pass filter comprises a plurality of resonance circuits each of which comprising an inductor formed by a core conductor and a resonance capacitor. The resonance capacitor comprises an internal electrode connected to the core conductor, and an internal ground electrode formed by internal layers and ceramics. In order to reduce the band in the band pass filter, the thickness of the ceramics between the ground electrodes formed on the upper and lower faces of the multilayer band pass filter and the core conductor may preferably be reduced or the spacing between the resonance circuits may be increased.
When the thickness of the ceramics between the ground electrodes formed on the upper and lower faces of the multilayer band pass filter and the core conductor is reduced, a further problem is raised in that the loss increases. When the spacing between the resonance circuits is increased, the transverse width of the multilayer band pass filter increases. This raises a further problem in that the whole dimensions of the multilayer band pass filter is increased. In other words, only the band of the band pass filter cannot be changed.
The present invention has a further object to provide a method of controlling the band of the multilayer band pass filter without increase of the loss and also without increase of the whole configuration of the multilayer band pass filter.